In recent years, fine ceramic materials, such as silicon nitride (Si.sub.3 N.sub.4) and silicon carbide have become increasingly practical in use as structural materials for high temperature-resistant applications or machine tools for their high strength and abrasion resistance in a wide range of high temperatures and their chemical stability. It is, however, an obstacle to a wide range of applications that these ceramic materials have the drawback of brittleness.
A number of attempts have been heretofore made to overcome the drawbacks of brittleness of the ceramic. One approach made successfully is to join a member of ceramic to a member of metal to form a ceramic-metal compound member. To improve such a ceramic-metal compound member, methods of providing strong joining strength between the pieces of ceramic and metal are now expected.
One approach to the drawbacks is to use a joining sheet which has a layer of brazing material and a layer of active metal, such as titanium, laminated on at least one surface of the brazing material layer. The joining sheet is put between a ceramic member and a metal member with the active metal layer in contact with a surface of the metal member and heat-treated to join the ceramic and metal materials together. Such a joining sheet is known from, for example, Japanese Unexamined Patent Publication No. 60(1985)-81071 entitled "Ceramics Joining Metal Sheet", laid open May 9, 1985.
A compound member of ceramic and metal members joined together by the use of the joining metal sheet described in the publication has a joining strength, at most about 35 to 40 kg/mm.sup.2 in a four-point bending test, which joining strength is not always sufficient to use the compound member in a wide range of applications.